The gas plasma vapor etching process has been utilized in the past for performing etching operations on semiconductor wafers by exposing the wafers to an active plasma to remove portions of materials carried by the semiconductor structure. The extension of reactive plasma etching beyond the patterning of silicon, silicon nitride and silicon oxide has greatly expanded the horizons of plasma etchings to include, among other things, the etching of aluminum in the production of small geometry integrated circuits. Plasma as compared to chemical etching produces better edge definition, less undercutting, considerably less photoresist adhesion sensitivity, and the elimination of so-called "knee breakdown" due to thinning of the photoresist at sharp edges. This thinning where the aluminum goes over and down the side wall of a cut leads to premature resist failure during wet etching, thus permitting removal of the metal at the near edge.
Although plasmas are not clearly understood, it is known that a special form of chemical materials can be made by exposing the compounds to high energy radio frequencies. Under the influence of these radio frequencies, compounds breakdown and rearrange to form transitory species with life spans so short that they are difficult to identify. Accordingly, unexpected reactions can be effected in a plasma that are difficult or impossible to effect using more conventional techniques. For example, a plasma of a very inert gas such as a fluorocarbon, known commercially as Freon, will etch glass, indicating that an active fluorine species is present in the plasma. In addition to the active chemical species, there are strong radiations, such as ultraviolet, and strong ion and electron bombardment of the surfaces within the plasma.
One of the great difficulties in plasma etching is the failure to achieve uniform etching of the wafer. As noted in U.S. Pat. No. 3,879,597, the edges of wafers are etched more deeply than the centers which results in a lack of uniformity of etching across individual wafers. This was partially remedied by employing slower etching rates, which cause less attack on the resist and by using greater spacing between the wafers. The referenced patent also employs a perforated inner chamber which partially prevents photoresist attack.
It has further been noted that beyond non-uniformity within each wafer, there remains a lack of etching uniformity from wafer to wafer wherein the etching rate decreases as the distance from the center of the circular electrode radially increases. As disclosed in an article entitled PLASMA ETCHING OF ALUMINUM by Herndon et al. at the M.I.T. Lincoln Laboratory, rotation of the wafer holder is carried out in order to "obtain consistent wafer-to-wafer and across-wafer etch times within a run and from run-to-run." It has been found, however, that even with wafer rotation, wafer-to-wafer etching rates vary causing uneven and unpredictable results in semiconductor fabrication.